Recently, demand for steel materials having improved corrosion resistance suitable for use in building and construction of chemical plants has been growing because of increased acidification of the atmosphere, more active marine construction, etc. Also materials having improved corrosion reistance are wanted for use in manufacturing internal combustion engine exhaust gas treatment apparatuses, because the practice of spreading salts on roads in snowy districts to prevent freezing has led to a new corrosion problem.
For these purposes, hot-dip aluminum-coated steel sheets, hot-dip zinc-coated steel sheets and hot-dip zinc-aluminum-alloy-coated steel sheets are widely used at present. However, these materials are not satisfactory. Hot-dip aluminum-coated steel sheets are severely corroded at the spots where the aluminum coating layer has cracked by heavy working, although the flat parts have excellent corrosion resistance. Usually hot-dip aluminum-coated steel sheets are prepared by using a molten aluminum coating bath containing 5.about.13% by weight of silicon in order to inhibit growth of an alloyed layer which impairs workability of the coated sheets and their coating layer consists of a 2.about.3.mu. thick Al-Fe-Si ternary alloy layer and an Al-Si alloy coating layer. This ternary alloy layer is very hard and brittle and easily suffers cracking when the coated sheets are heavily worked. Stress concentrates at the cracked spots of the ternary alloy layer, which induces cracking of the outer coating layer. In the case of hot-dip aluminum-coated steel sheets, aluminum hardly exhibits sacrificial corrosion effect to for the steel substrate in the atmospheric corrosion environment and, therefore, the steel substrate is corroded at the locally-exposed spots formed by severe working.
In the case of hot-dip zinc-coated steel sheets or hot-dip Zn-Al-alloy-plated steel sheets, the steel substrate is also exposed when they undergo severe working and the Zn or Zn-Al-alloy coating layer cracks. In this case, the sacrificial corrosion effect of zinc is produced between the locally exposed steel substrate and the coating layer and thus the corrosion of the steel substrate is prevented to some extent. However, the Zn coating layer or the Zn-Al coating layer is corroded more rapidly in the vicinity of the exposed spots of the steel substrate than in the flat parts and thus eventually the corrosion of the steel substrate proceeds very rapidly.
Therefore, it is necessary to improve the corrosion resistance of the steel substrate per se in order to enhance corrosion resistance of the exposed parts of the steel substrate of these hot-dip metal-coated steel sheets. It is well known to add Si and Cr to steel singly or in combination in order to improve the corrosion resistance of substrate steel mateials. However, when steel substrates containing Si and/or Cr are hot-dip-coated with Zn, Al or a Zn-Al alloy by a continuous hot-dip coating line provided with a non-oxidizing pretreatment furnace (a Szendimir apparatus for instance), Si and/or Cr in the steel concentrates to the surface forming oxides during the annealing step preceding the coating in the continuous coating line and impairs the wettability of the substrate sheets, which results in occurrence of a plurality of dewetted (non-coated) spots, which become starting points of corrosion. Therefore, incorporation of these elements eventually deteriorates the corrosion resistance of the coated steel sheets.
In compensation for the defect caused by addition of these elements, it has been proposed to electrolytically plate the steel substrate containing Si and/or Cr with Ni before hot-dip metal coating in order to prevent the concentration of Si and/or Cr at the surface which occurs during the annealing step (JP-A-60-262950, 61-147865).
However, this pre-plating with Ni is not only expensive but the Ni tends to diffuse into the coating layer and deteriorate the corrosion resistance of the coating layer per se.
We carried out an extensive study for overcoming the defect of the prior art hot-dip coating, and have found that the occurrence of the dewetted (non-coated) spots is prevented by electrolytically plating substrate steel sheets with an Fe-B (iron-boron) alloy containing a small amount of B prior to the hot-dip metal coating, and thus hot-dip-coated steel sheets having excellent corrosion resistance can be produced.